1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor having improved temperature characteristics.
2. Description of the Background Art
In FIGS. 1 and 2, there is shown a conventional semiconductor acceleration sensor which is fabricated by carrying out an etching process an N-type silicon substrate having a (100) surface, as disclosed in Japanese Patent Laid-Open Specification No. 62-213280. In semiconductor acceleration sensor prepared by etching the silicon substrate, a rectangular silicon center mass or weight 2 is arranged in the central portion of a frame 1 isolated therefrom, and a cantilevered beam 3 for detecting an acceleration couples the central left side portion of the silicon weight 2 to the frame 1. Another cantilevered beam 4 for a temperature compensation having no weight on its end is formed to the frame 1 near the cantilevered beam 3 in parallel therewith. The cantilevered beam 4 is so designed that the thickness and width are equal to those of the cantilevered beam 3. A pair of piezoresistors 5a is formed in the surface portion of the cantilevered beam 3 by doping a P-type impurity in the direction of the &lt;110&gt;crystal axis so as to extend in parallel with each other in the longitudinal direction of the cantilevered beam 3. A pair of piezoresistors 5b are formed in the surface portion of the cantilevered beam 4 in the same manner as those of the cantilevered beam 3. Usually, a protecting film composed of SiO.sub.2 is formed over the surface of the silicon substrate including the piezoresistors 5a and 5b.
The piezoresistors 5a and 5b are so connected to constitute a bridge structure shown in FIG. 2, and thus, in fact, the ends of the piezoresistors 5a and 5b are bonded to electrodes (not shown) composed of an aluminum film or the like in order to connect exterior devices via wirings.
Now, when acceleration is provided for the semiconductor acceleration sensor, as indicated by arrows in FIG. 1, the silicon weight 2 receives the force due to acceleration, and thus the cantilevered beam 3 for acceleration sensing is bent. Hence, a bending stress is applied to the piezoresistors 5a formed in the surface portion of the cantilevered beam 3, and their resistances are varied. At the same time, in turn, the force which the cantilevered beam 4 for temperature compensation receives due to acceleration is slight, and the cantilevered beam 4 is hardly bent. Consequently, a bending stress is hardly applied to the piezoresistors 5b, and their resistances are hardly changed.
Assuming that the resistance of each of piezoresistors 5a 5b is defined as R and the resistance variation of the piezoresistor 5a is .DELTA.R, the output voltage Vout of the bridge circuit shown in FIG. 2 is represented by the following formula. EQU Vout=(1/2).multidot.(.DELTA.R/R).multidot.V.sub.B (1)
wherein V.sub.B is the voltage applied to the bridge circuit.
Further, consideration as to offset temperature characteristics is sufficiently provided in the semiconductor acceleration sensor described above. That is, the thermal stress caused by the difference of the thermal expansion coefficient between the silicon and the protecting film such as SiO.sub.2 formed thereon is different among the four piezoresistors 5a and 5b to cause an offset voltage. The influence due to the generation of the offset voltage can be prevented by using the two cantilevered beams 3 and 4 for acceleration sensing and temperature compensation. That is, these two cantilevered beams 3 and 4 are designed to equal conditions such as thickness and width except that the acceleration sensing cantilevered beam 3 has the silicon weight 2 on the end, and the piezoresistors 5a and 5b of the cantilevered beams 3 and 4 are also designed to equal conditions and layouts. Therefore, almost the same thermal stress caused by the difference of the thermal expansion coefficient between the protecting film such as SiO.sub.2 and the silicon is provided for the piezoresistors 5a and 5b, and thus the offset temperature characteristics are improved.
However, the conventional acceleration sensor has some problems. That is, firstly, since the two cantilevered beams 3 and 4 and weight 2 connected to the cantilevered beam 3 are formed within the frame 1 having a rectangular ring form with a space 6 therebetween, a complicated etching process is required.
Secondly, in addition to the thermal stress caused by the difference of the thermal expansion coefficient between the protecting film and the silicon, there is another main cause for reducing the offset temperature characteristics. That is, usually, the frame 1 is adhered to a mounting member, and the mounting member is attached to a package by using an adhesive to carry out a mounting of the sensor chip. There is another thermal stress caused by the difference of the thermal expansion coefficient among the silicon, the mounting member, the package and/or the adhesive. However, no consideration relating to the latter thermal stress is provided in the conventional acceleration sensor shown in FIG. 1, and hence thermal stresses of different strengths are applied to the piezoresistors. In particular, thermal stresses of different strengths are applied to the piezoresistors 5a and 5b for acceleration sensing and temperature compensation, with the result of reducing the offset temperature characteristics.
Thirdly, since the two cantilevered beams 3 and 4 are attached to a part of the inside side surface of the frame 1, stress concentrated areas or portions indicated by circles 7 are produced in the acceleration sensor to cause a weak structure for breakdown or destruction.